(1) Field of the Invention
The present invention relates generally to the fabrication of color light detectors. More particularly, the present invention relates to the formation of protective coatings on filters used in color light detectors.
(2) Description of Related Art
Color light detectors are becoming increasingly important in digital imaging applications. Digital imaging systems often use arrays of photo-detectors to generate an image of a subject. In order to generate a color digital image, color filters are fabricated over light sensors such as photo-detectors. Each filter allows only a predetermined color of light to reach a corresponding photo-detector thus determining what color light will be sensed by the photo-detector. By grouping sets of light sensors together, the intensity and color of light reaching an area can be determined.
Each color pixel in a digital image is typically generated by combining the output of a group or set containing several photo-detectors. In one implementation, the output of three corresponding photo-detectors are combined to generate one color pixel. Each corresponding photo-detector is located in close proximity to the other two corresponding photo-detectors. Each corresponding photo-detector has a different color filter filtering received light. In one example, a blue color filter, a red color filter and a green color filter may each be used over a corresponding photo-detector. By determining the intensity of light passing through each color filter, the intensity of light of a particular color or wavelength can be determined. An electronic processor interpolates the data from the three photo-detectors and combines them to determine the color of light received by the photo-detectors in the general region of the pixel. This information is processed electronically and combined with other sets of photo-detectors to generate a digital color image.
Photo-detectors and color filters are typically formed in a complimentary metal oxide semiconductor (CMOS) fabrication process. A number of effects occur during the fabrication process which reduce the filtering capability or damage the color filters. In particular, three problems faced by the fabrication process are described in the following three paragraphs.
A first problem which results from the fabrication of color filters is color bleeding of compounds from adjacent color filters. Color filter arrays which are placed over the photo-detectors or imaging sensors are often generated by depositing pigment dispersed polymer films. The type of pigment determines the filtering capability of the filter. In a typical color detection system, adjacent filters thus have different pigments. The performance of the system is optimized when each photo diode is covered with a single color filter, whether it be red, blue or green. The filter blocks other colors from passing through the filter. Ideally, it is desirable to fabricate filters which transmit one hundred percent of the light at a predetermined frequency range, and completely block light transmission outside of the predetermined range of frequencies. Thus ideally, pigments which determine what color of light will pass through a filter is preferably completely confined to a filter and does not xe2x80x9cbleedxe2x80x9d into adjacent filters. In practice, the contact between the various different color filters (red, blue, green) results in a slight intermixing of pigments (xe2x80x9cbleedxe2x80x9d) during the fabrication process. The bleed results in a broadening of each individual filter response reducing the color delineation capabilities of each filter. This bleed degrades the overall performance of the system.
A second problem with current fabrication techniques is that during a grinding and gold deposition process (grind/gold process), color filters are often damaged. After the final deposition of CFA (filter) layer materials, wafers or substrates containing the color filters are transferred to a grind/gold process where a protective front side tape is applied to the wafer while the backside of the wafer is thinned and coated with gold. After completion of the grind/gold process, the tape is removed in a detaping operation. Due to the polymeric nature of the filter material (CFA material), the filters are vulnerable to physical damage during the detaping operation. Damage to the color filters jeopardizes the functionality of the fabricated light detector.
A third problem with current methods of fabricating filters in CMOS processes results from repeating processing steps on the entire filter set each time a filter of a different color is added. After generation of a first color filter, each subsequent color in the filter set is produced by a subsequent deposition and photo definition of pigment dispersed in polymer films. Thus a three color set (red, green and blue) of color filters involves three depositions of pigments. Each filter is composed of a CFA layer. Each CFA layer is manufactured from the same starting materials. In the prior art, no physical or chemical resistant barriers are used between the layers. Thus misprocessing or process excursion in the working or processing of a layer may damage previously created filters and require reworking of previously deposited filter layers.
Thus a method of protecting each filter as it is generated is desirable. In particular, the method would preferably utilize a barrier to protect each individual filter in a multicolor filter array such that the barrier would prevent bleeding between adjacent color filters. The barrier would preferably be non polymeric to help prevent damage during the detape process. The procedure would also preferably be performed after each CFA layer is deposited creating a barrier between layers. The barrier allows accidental misprocessing or minor processing variations to occur without damage to previously fabricated layers reducing the probability that a rework of previously deposited layers is necessary. Such a technique for generating a protective barrier is described in the following application.
A structure for detecting light is described. The structure includes first color filter deposited over a light sensor. A protective polymer silation layer is deposited over the first color filter.